The present invention relates to a process for enhancing the sulfur capture capability of an alkaline earth material.
More particularly, the present invention relates to a process for enhancing an alkaline earth material by treatment with an aqueous treatment solution containing an inorganic salt and heating the treated material in the presence of a gas containing SO2 to remove the SO2.
A wide variety of methods are currently employed to remove sulfur compounds from gaseous streams. Hydrogen sulfide, arising, for example, from coal gasification operations, is removed to purify reducing gases for other uses downstream, such as further chemical reaction. Sulfur oxides and hydrogen sulfides, when present, must be removed from exhaust gases to maintain environmental air quality. Sulfur oxides arise from burning carbonaceous, sulfur-bearing fuels such as coal or heavy oil. It has long been known that the calcium in limestone and the calcium and magnesium in dolomite are effective sulfur capture agents. The stone may be pre-calcined to convert the calcium or magnesium carbonate to the oxide, which reacts with sulfur compounds to produce in the first instance, calcium sulfide from hydrogen sulfide gas, or in the second instance, calcium sulfate from sulfur oxides and oxygen. Elevated temperature is generally required for the reactions in the dry state and the calcination reaction (release of CO2) may take place simultaneously with the absorption reaction. Wet scrubbing methods may be used effectively to remove sulfur compounds in gases at lower temperature.
Limestone and dolomite sorbents for sulfur oxide find particular application in fluidized bed combustion, which is a technology for industrial steam generation or electrical power generation. The limestone or dolomite is used in the form of crushed granules, irregular in shape. The limestone or dolomite granules are mixed directly in the fluidized bed with sulfur containing fuel. An upward flow of air supplies oxygen for burning the fuel and fluidizing the bed. Sulfur contained in the fuel is oxidized and subsequently absorbed by the limestone in a high temperature, dry state reaction. Spent sorbent and ash are removed from the bed continuously as fresh limestone and fuel are added.
Efforts have been made to improve the reactivity of the limestone or dolomite sorbent as increased reactivity would bring cost savings in that less limestone could be used to achieve the desired sulfur reduction. However, the approaches which have been proposed heretofore have often led to unacceptably high cost increases which outweigh the cost savings of reduced limestone usage. Accordingly, the need still exists for an approach to sulfur reduction by sorbent which increases the reactivity of the sorbent in an economically acceptable manner.
The present invention provides, according to one aspect thereof, a computer controlled process for reducing SO2 from a flue gas. The process includes the basic steps of providing an aqueous treatment solution containing an inorganic salt and a means for introducing the aqueous treatment solution containing an inorganic salt and controlling the introducing means in response to a computer program to effect introduction of the aqueous treatment solution into contact with an alkaline earth material. Thereafter, the process includes heating the alkaline earth material in the presence of the flue gas containing SO2 to remove the SO2.
According to one feature of the process of the present invention, the alkaline earth material is limestone or dolomite. According to another feature, the inorganic salt is selected from a group of thermally decomposable compounds of sodium including sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium nitrate, and sodium acetate.
In selected applications of the process of the present invention, the concentration of the aqueous treatment solution is adjusted to yield a net concentration of between 0.05 to 5 percent inorganic salt in the treated limestone or dolomite.